1905.] on High Power Microscopy. 13 



tage to produce dark outlines on a bright field or bright outlines on a 

 dark field. Since neighbouring beams of hght can thus be made to 

 take diiferent paths it is quite possible to an-ange the eye-piece so 

 that it may catch the one and miss the other beam ; and, according as 

 the one rejected comes from the margin of the object or from the 

 background, so the image ^Yill be outlined dark or outlined bright, in 

 either case in contrast with the field. The effects are known as bright 

 field or dark field illumination, the case of dark field illumination 

 being exactly analogous to the effect obtained by artists by means of 

 cross lighting. The most universally familiar example of cross fight- 

 ing is the gibbous or crescent moon, upon the surface of which the 

 mountain tops and salient points stand out from a sea of shadow. 

 Fig. 4 is a diatom shown under dark field illumination, the instrument 

 being so arranged that the light from the field escapes the eye-piece, 

 while the light refracted by the silex of the valve is transmitted to 

 the eye-piece and furnishes the picture seen in the photograph. The 

 reciprocal case of briofht field illumination is illustrated by Fig. 2. 



The application of this mode of producing a picture is evidently 

 subject to this limitation, that light can be thrown upon the object 

 under an angle so wide that some of it shall reach the eye-piece and 

 some of it shall travel out of the instrument altogether. Hence 

 it is difficult of application when objectives of very wide angle are 

 used, and when the angle of the condenser which suppUes the Ught is 

 less than that of the objective which receives it the method of 

 dark field illumination cannot be applied at aU. This Umitation has 

 long been felt to be a considerable drawback in connection with the 

 use of high power lenses and especially of immersion lenses, which 

 always have great angular grasp. The problem thus presented, of 

 devising a system of dark field illumination which shall be appUcable 

 to object glasses of the widest possible aperture, has recently been 

 attacked, and with notable success, by Dr. Siedentopf, who has 

 succeeded by its means in rendering visible objects so minute and 

 clustered so close to one another that by no other known contrivance 

 can they be rendered separately visible at all. Thus he has demon- 

 strated — to take one example only — the minute particles of gold 

 which, disseminated through a piece of glass, impart to it by their 

 combined effect a ruby colour. Viewed under the highest magnifying 

 power and under any ordinary illumination these massed particles 

 of gold would appear at best as nothing more definite than a luminous 

 haze. But by means of the Siedentopf apparatus the illuminating 

 beam is thrown in at the side of the specimen and crosses the field 

 of view at right angles to the axis of the instrument, so that if it were 

 not for the reflection and refraction of the light at very wide angles 

 to its original direction no single ray would find its way to the eye 

 of the observer looking down the instrument. But even this would 

 not suffice to bring individual particles of disseminated gold into view 

 if any considerable thickness of the specimen were so lighted up, for 



